The sinus node, where present, intact, and functioning, is the ideal and natural sensor for controlling the heart rate. Often, however, the conduction pathway from the upper chambers (atria) to the lower chambers (ventricles) is blocked, that is, the depolarization signal from the atria cannot reach the ventricles. In such cases, atrial synchronous (VDD) pacing is the only type of pacing required to restore an adequate heart rate. Such pacemakers function as an artificial conduction pathway to reestablish AV synchrony.
VDD pacemakers were first disclosed in U.S. Pat. No. 3,648,707 issued to Wilson Greatbatch, on Mar. 14, 1972, incorporated herein by reference in its entirety. A standard VDD pacemaker requires two electrodes for sensing depolarization signals one within the right atrium and a second in the right ventricle. In general, a standard VDD pacemaker has two sense amplifiers, one for the atrium and one for the ventricle—but only one pacing output circuit for the ventricle. Also included is an A-V interval timer, which starts an AV delay in response to the sensing of an atrial depolarization. On expiration of the A-V interval, a ventricular pacing pulse is triggered, unless inhibited by a sensed R-wave. A sensed or paced R-wave starts a lower rate timer. If this timer expires (that is, without another atrial depolarization being sensed), it triggers a ventricular pacing pulse. The Greatbatch pacemaker also included a ventricular upper rate timer, which prevented or blocked ventricular pacing until the upper rate timeout had expired. In other words, the pacemaker would only pace the ventricle in synchrony with the atrium up to a predetermined upper tracking rate, corresponding to the upper rate interval. If the atrial rate exceeded this rate, the pacing rate would fall to the higher of ½ of the sensed atrial rate or the lower rate.
U.S. Pat. No. 4,059,116 issued to Adams on Jan. 12, 1976, also incorporated herein by reference in its entirety, included an improvement to the pacemaker disclosed in Greatbatch. In the Adams pacemaker, rather than blocking a ventricular stimulus in response to the timeout of the A-V interval during the upper rate interval, generation of a stimulus pulse was delayed until the end of the upper rate interval. That is, the AV interval was extended until the end of the upper rate interval. In addition, a post-ventricular atrial refractory period was included, which specified a period of time following a ventricular pacing pulse or a sensed ventricular contraction, during which an atrial contraction would not be sensed and, thus, could not start an AV interval. The net result of these two additional features was to produce a pacemaker that did not display such an abrupt drop in average ventricular pacing rate in response to an intrinsic atrial rate that exceeded the rate defined by the upper rate interval. The behavior of the Adams pacemaker resembled the natural physiologic condition known as Wenckebach rhythm. In commercially marketed pacemakers employing the Adams invention, the behavior of the pacemaker in the presence of high natural atrial rates is referred to as “Psuedo-Wenckebach” or “Pacemaker-Wenckebach” upper rate behavior.
This standard VDD pacemaker had another issue with regard to the timing. At first, VDD pacemakers had a “fixed” Post Ventricular Atrial Refractory Period (PVARP) following a Premature Ventricular Contraction (PVC). Since PVCs are often followed by a retrograde atrial depolarization, these events, if sensed outside of the fixed PVARP, would start a Pacemaker Mediated Tachycardia (PMT). U.S. Pat. No. 5,103,820 issued to Markowitz in 1992, incorporated herein by reference in its entirety, described a method for programming a programmable period for the PVARP. In addition, this patent also provided a means of automatically extending the PVARP after PVCs that effectively blocks an atrial depolarization from starting an AV interval. As a result, PMTs resulting from retrograde atrial depolarizations after such PVCs virtually ceased to be an issue.
Throughout this period of time, the VDD pacemakers known in the art up to 1992 required two leads, one in the atrium and a second in the ventricle. Implanting two leads extended the time for the implant procedure, as well as the possibility for complications during and post implantation. This was particularly true of the atrial lead, which is more prone to dislodgement. Such dislodgement, when it occurred, reduced the operation of the VDD pacemaker to that of a standard VVI pacemaker. Moreover, the use of two leads increased the cost of the VDD pacing system.
U.S. Pat. No. 5,172,694 issued to Flammang in 1992 and U.S. Pat. No. 5,454,836 issued to Van der Veen in 1995, both incorporated herein by reference in their entirety, describe the use of a single lead for atrial sensing and ventricular pacing. Specifically, it is a single dual chamber lead for providing reliable ventricular sensing and pacing from a lead in the right ventricle and atrial sensing from at least two locations in the atrium to determine P-wave direction and conduction time. While this “single pass” VDD lead represented an advance in the art, it unfortunately was often difficult to use. Although the implantation process resembled that of a single standard ventricular lead, positioning the “floating” electrodes within the atrium to provide adequate sensing was often difficult. Moreover, once properly positioned, there was no guarantee that these electrodes would remain in the optimal position since there was no way to affix the electrodes to the wall or floor of the atrium. This new lead also was more difficult to manufacture and, as a result, did not significantly reduce the cost of the VDD pacing system.
A more recent development involves the addition of rate responsive (VDDR) pacing to the standard VDD pacing. U.S. Pat. No. 5,350,409 issued to Stoop, et al. in 1995 describes one such embodiment. The basis of VDDR pacing is to provide a ventricular rate via an artificial sensor (activity, accelerometer, minute ventilation, QT, etc.) to match the patient's physiologic needs in the absence of a healthy sinus node. The issue to be resolved is to ensure that the sensor-indicated rate does not conflict with the sinus rate. This Stoop patent discloses how the sensor-indicated rate is adjusted as a function of sensed sinus rate. The natural sinus rate is compared to the sensor-indicated rate at one or more rates, and the sensor response function is adjusted, or adapted as a function of such comparisons, so as to optimize sensing of the atrial rate and maintenance of synchronous operation. Again, however, the issue is whether or not the atrial depolarization wave has been sensed. Undersensing or loss of sensing on the atrial lead could potentially lead to a loss of AV synchrony.